Sodium carbonate perhydrate has been recognized to be a desirable component for a detergent composition because it is readily soluble in water, because it has a high active oxygen content, and because it also provides an inexpensive source of nonpolluting alkalinity. Pure sodium carbonate perhydrate conforms to the chemical formula 2Na.sub.2 CO.sub.3.3H.sub.2 O.sub.2 which contains 15.28% active oxygen (AO). However, sodium carbonate perhydrate generally requires a coating to protect it from decomposition when formulated into a detergent and stored in an open container in a laundry room.
Numerous processes have been proposed for manufacturing sodium carbonate perhydrate (SCP). One of the primary methods is a crystallization process in which aqueous solutions of hydrogen peroxide and sodium carbonate are mixed in a reactor and the SCP formed is filtered off. The product is usually salted out by the addition of sodium chloride or other suitable reagents. Such processes are disclosed in U.S. Pat. Nos. 2,380,620 and 2,541,733. While the crystallization process can produce a product with a bulk density of 900 kg/m.sup.3 or more and offers advantages such as good mixing and heat transfer, it has the disadvantage that there typically is a substantial loss of active oxygen in the mother liquor, so that low peroxygen efficiencies are obtained. That is, conversion of hydrogen peroxide utilized to active oxygen in the finished product is low.
In another method, taught by U.S. Pat. No. 3,555,696, SCP is made by a spray-drying process in which hydrogen peroxide solution is added immediately before atomization of a spray charge of sodium carbonate in a spray tower. Thereafter the product SCP is dried, yielding a very dusty product with a very low bulk density.
It is apparent from the prior art that large volumes of mother liquor are to be avoided. A process yielding high peroxygen efficiencies usually uses only a sufficiently large amount of water to act as a reaction medium and to provide a heat sink for the heat of reaction.
The desire to minimize the amount of water in the reaction system has led to exploration of the so-called "dry" method. However, when the reaction is carried out in the absence of a sufficient amount of water, the reaction is not efficient and decomposition losses are quite high. In the dry method, hydrogen peroxide is sprinkled directly onto sodium carbonate powder to form a moist mass, the mass is then dried. The procedure may be repeated to build up the oxygen content of the perhydrate. Attempts to operate such a process have produced only unsatisfactory perhydrate products with a low bulk density and thus the process is not in commercial use insofar as is known. Typical dry processes are taught in U.S. Pat. No. 3,864,454, in which it is necessary to dry the product in carbon dioxide and in European Patent Application 0070711, in which the reaction mixture is maintained in a vacuum before drying. In accordance with East German patent 212,947, the product is so fine that a separate recycling granulation step is required. On the other hand, U.S. Pat. No. 4,171,280 avoids the heat sink problem by restricting the amount of hydrogen peroxide to provide a maximum active oxygen content of the product to 6%, thereby avoiding decomposition and caking of wet reaction mixtures.
The dry process for the formation of SCP has a basic deficiency, namely, the difficulty of proper heat transfer of the exotherms that are generated as a result of the reaction. Reaction between aqueous hydrogen peroxide and solid soda ash generates an exotherm in two ways: the heat of hydration of sodium carbonate with the water present in hydrogen peroxide, and the heat of perhydration, that is, the reaction of sodium carbonate with hydrogen peroxide to produce sodium carbonate perhydrate. Both these heats tend to increase the reaction temperature quite markedly, particularly in the absence of efficient mixing and/or cooling.
Dusting is another problem associated with the dry process. When finely divided soda ash is sprinkled with hydrogen peroxide solution and mixed very efficiently to dissipate the heat, a large amount of dust is formed. This results in low peroxygen efficiency and/or a product having low active oxygen values. On the other hand, if granular, dense soda ash is used, the dusting effects are less but the reaction becomes relatively inefficient. In either case, the product tends to agglomerate to form a product with a low bulk density.
A hybrid process combining the dry process and the wet process is taught by U.S. Pat. No. 3,860,694 in which anhydrous or hydrated sodium carbonate having a particle size distribution between U.S. Standard Sieve No. 14 and 325 is contacted with from 35% to 90% hydrogen peroxide, a magnesium stabilizer, and sufficient water to maintain the reaction mass moist. The moist reaction mass is reacted from 5 minutes to 3 hours. Subsequently the moist reaction is dried.
U.S. Pat. No. 4,970,058, teaches a sodium carbonate perhydrate process in which hydrogen peroxide, anhydrous sodium carbonate, and a diphosphonic acid are reacted to make a composition in which sufficient anhydrous sodium carbonate is present to react with any water either already present in the composition, or any water which may be formed from the hydrogen peroxide. The diphosphonic acid appears to permit any water present during the manufacture from being retained as sodium carbonate monohydrate. The process provides high peroxygen efficiency, low dusting and the product is very stable on storage. Its only apparent disadvantage is the maximum active oxygen concentration is about 11.2%. It is desirable to have a product with an active oxygen content higher than 11.2%.
U.S. Pat. No. 5,045,296 teaches a process for SCP by uniformly distributing aqueous 50% to 75% hydrogen peroxide and 11/2% to 13% by weight of a diphosphonic acid onto a dry, particulate reaction mixture of anhydrous sodium carbonate with a particle size distribution between 300 and 74 micrometers. The process concurrently balances the heats of hydration and of perhydration of sodium carbonate and the heat of evaporation of water to maintain the reaction mixture between 50.degree. C. and 80.degree. C. to evaporate substantially all of the water from the resulting reaction mixture and cooling the resulting reaction mixture to provide said product as a free-flowing, stable, granular material with a particle size distribution substantially the same as the anhydrous sodium carbonate, and containing between 13% and 141/2% active oxygen.
Coating SCP to minimize decomposition when formulated into a detergent not only has the obvious disadvantage of diluting the overall active oxygen content, but also has added disadvantages. These disadvantages include reducing the bulk density because of agglomerating the particles and retarding the rate of release of active oxygen into the solution.
It has been suggested that particles of peroxygen compounds be coated by compounds, such as trona (U.S. Pat. No. 4,105,827); sodium silicate (U.S. Pat. No. 3,951,838); sodium perborate plus sodium silicate (U.S. Pat. No. 4,194,025); boric acid (U.S. Pat. No. 4,321,301); wax (U.S. Pat. No. 4,421,669); a polymer latex (U.S. Pat. No. 4,759,956); sodium silicate plus a chelate (U.S. Pat. No. 4,117,087); and wax plus a fatty acid (U.S. Pat. No. 4,126,717). Many of these treatments show some improvement in short term storage stability in a humid environment. Those few coated SCP products that were stable when formulated into a dry household laundry detergent were found to release their active oxygen when added to water too slowly to be of value in a laundry detergent formulation.
U.S. patent application Ser. No. 5,194,176 filed Apr. 2, 1991 teaches a storage-stable compound of 45% to 75% sodium carbonate perhydrate, 0.1% to 3% diphosphonic acid or salt and anhydrous sodium carbonate coated with sodium borosilicate.
The composition is storage-stable, but has the disadvantage of a low assay.